Traditionally, earthen boreholes for oil and gas production, fluid injection, etc., frequently referred to as “wells,” are drilled by rotating a drillstring from the drilling rig, by means of a rotary table and kelly. The drill bit on the lowermost end of the drillstring is in turn rotated, and with the addition of weight applied to the drill bit by drill collars and other components of the drillstring, drilling takes place.
An alternative way of rotating the drill bit is by use of a downhole device, either a downhole motor, such as a positive displacement motor (frequently called a Moineau motor) or a downhole turbine. A “downhole motor.” as used herein, encompasses any arrangement that generates rotation of a tool (e.g., a drill bit) and which is positioned downhole in a workstring (e.g., a drillstring). Generally, when a downhole motor is used, the workstring is not rotated. However, in some implementations, the workstring can be rotated slowly to reduce drag on the workstring caused by friction with the wellbore. Downhole motors utilize circulation of a fluid (e.g., drilling fluid, “mud,” or in some cases gas), down through the workstring and through the downhole motor, to generate rotation. Downhole motors are also used in settings other than conventional drilling, for example with coiled tubing, or workstrings used in well cleanout work and the like.
On occasion, circumstances can arise which cause a lower section of the downhole motor, referred to as the bearing assembly, to become separated from an upper section of the downhole motor, referred to as the power section. This separation may be caused by mechanical failure or by a threaded connection becoming unthreaded, also referred to as “backing off.” A threaded connection generally can fail or back off after a stall has occurred in a downhole motor.
As an example, during performance of a downhole operation, the downhole motor may stall. When this occurs, the pressure of the circulated fluid increases, alerting the operator of a problem. To address the issue, the operator typically will reduce or eliminate the amount of downward weight by lifting the workstring or coil tubing string. However, if the motor comes out of the stall very suddenly, reaction forces generated can cause a threaded connection to become loose and back off. Once this occurs, the bearing assembly with the rotor separates from the stator of the power section and will remain in the hole. This necessitates a retrieval operation, known as “fishing,” to remove the bearing assembly and rotor from the wellbore. Because fishing is a very expensive and time consuming process, it should be avoided if possible. Moreover, using a fishing technique to fish a motor from the wellbore can be challenging because rotors typically are helically fluted and hard-chrome coated, making them difficult to latch onto with conventional fishing tools.